Control apparatus



Aug. 1940- J. w. DAWSON 2.210.709

CONTROL APPARATUS Filed May 7, 1938 2 Sheets-Sheet 1 /2 j INVENTOR John 14/. Dan/50x7.

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202 NEW) J. W. mwssw CONTROL APPARATUS Filed May '7, 1938 2 Sheets-Sheet 2 INVENTOR Jbn h/flam/sfor) WITNESSES:

Patented Aug. 6, 1940 UNITED STATES 2,210,709 CONTROL APPARATUS John W. Dawson, Auburndale, Mass, assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application May 7, 1938, Serial No. 206,642

14 Claims.

My invention relates to control apparatus and has particular relation to apparatus for controlling resistance welding systems.

A welding system of the type in use in industry at present ordinarily comprises a pair of welding electrodes, one or both of which are urged into engagement with the material to be welded by the application of fluid pressure. After the electrodes are thus engaged with the material, welding current is applied. The welding current flows for a short interval oftime and is then discontinued. After this the electrodes are disengaged from the material by exhausting the fluid producing the pressure and the material is advanced in preparation for another welding operation.

In welding apparatus of the type just described, it is, of course, essential that the electrodes shall remain in engagement with the material to be welded until the welding current has completely ceased to flow and the melted or softened metal at the weld has chilled and hardened. If the welding electrodes are disengaged from the material while welding current is still flowing, arcing takes place and the material. is badly burned. If the welding electrodes are disengaged while the metal is still soft the stresses present in the welded material partially rupture the weld metal and cause weak welds. While some delay is thus necessary it is essential that the welding progress at the greatest possible speed and therefore the minimum time consistent with the production of high quality welds should be consumed in electrode dwell after a welding operation.

In accordance with the teachings of the prior art, the problem involved in this connection was solved by providing a switching mechanism responsive to the flow of the welding current pulses for the fluid-pressure control valve. The switch- I provide apparatus for controlling a welding system wherein the welding electrodes will be engaged with and disengaged from the material to be welded at the precisely proper time regardless of the length of the welding pulses.

Another object of my invention is to provide a welding system incorporating fluid pressure means for moving the electrodes into and out of engagement with the material to be welded wherein the valve provided for the application and release of the fluid pressure shall be so controlled that the electrodes shall at all times be moved into and out of engagement with the material to be welded at precisely the stant.

A further object of my invention is'to provide a control system incorporating a plurality of electromagnetic relay elements that shall so cooper ate that the actuation of one of said elements shall depend in a precise manner on the flow of current pulses through another of the elements.

More concisely stated, it is an object of my invention to provide a resistance welding system in which the current flowing through the material to be welded shall be of precisely the desird magnitude and shall persist for a precisely predetermined time interval and the welding electrodes shall remain in engagement with the metal for just suflicient time to enable the metal to properly chill and harden.

According to my invention, the fluid pressure valve is electromagnetically actuated. The movement of the welding electrodes relative to the material to be welded is precisely controlled by energizing and deenergizing the valve solenoid at the proper instants. For this purpose, a time delay relay is provided. 'The relay requires uninterrupted actuation for a predetermined interval'of time before it operates; the interval is subproper instantially longer than the time required for actuation of an ordinary quick-acting electromagnetic relay. The actuation of the time delay relay begins just after the pressure valve is operated and permits the application of pressure to move the welding electrodes into engagement with the material to be welded. After this a welding pulse is applied. If the welding pulse is of relatively short duration, no further action takes place during or after the welding current flows. The time delay relay merely continues to be actuated and after its characteristic time expires, it operates and, in turn, causes the valve to operate to release the fluid pressure and to permit the welding electrode to disengage the material. Since the characteristic time period of the timedelay relay is substantially longer than the interval during which a welding current pulse of the length just mentioned flows, the disengagement of the electrode from the material takes place a substantial interval of time .after the current flow has ceased.

If, on the. other hand, the welding current pulse is of substantial length, another relay is operated by the flow of the welding current. The latter relay causes the actuation of the time delay relay to be interrupted. Since the latter relay remains in its operated condition as long as the welding current pulse now flows, the time delay relay' remains in an unactuated condition until the current pulse has ceased flowing. When the welding current has ceased to flow, the second relay is deenergized and the actuation of the time delay relay again commences. The latter now operates at the end of its characteristic time period causing the movable welding electrode to disengage the material. In this case, the disengagement again takes place after the flow of welding current has ceased.

It is seen that the time delay relay as used has two functions; it remedies the situation which arises in the prior art arrangement when the welding time is so short that the-relay across the welding load fails to operate and it provides for a definite minimum period of dwell. This period is substantially the same in all cases, i. e., it is the characteristic time of the time delay relay. The maximum deviation which may arise in this period is less than the time required for therelay across the welding load to pick up.

The novel features that I consider characteristic of my invention are set forth with particu larity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof will best be un-' derstood from the following description of a specific embodiment when read in connection with the accompanying'drawings, in which weldedin and out of engagement with the mate-' rial. The movement of the latter electrode takes place under the action of fiuid pressure, preferably compressed air.

The flow of the compressed air is controlled by an electromagnetically actuated valve I3 held closed by a spring I5 and opened by the action of a solenoid I1. When the valve I3 is opened, air fiows into a piston chamber l9 and there'causes a piston 2| carrying the movable welding elec-' trode 9 to move upwardly against the action of another spring 23 carrying the electrode into engagem'ent with the work II. After the movable electrode 9 has engaged the work, the back pres-' sure of the air'actuates a back switch 25 by operating on a second piston 21 carrying the movable contactor 29 of the switch and the latter initiates a welding operation.

, The primary 3| of the welding transformer 3 is supplied from an alternating current source 33 of the ordinary commercial SCI-cycle type. The power is preferably supplied through a valve system comprising a pair of electric discharge devices 35 and 31 of the mercury-pool immersedig'nition-electrode type connected in anti-parallel. To properly time the flow of current, the

supply of initiating impulses to the'ignitio'n elec trodes 39 of the discharge devices is controlled through a system including an auxiliary discharge device lI for starting the current fiow,"an auxiliary discharge device 93 for stopping the current flow and a pair of initiating auxiliary electric discharge devices .45 and. M through which current flows to the initiating electrodes.

When the current pulse supplied to the material II to be welded is relatively short, the time delay relay 39 which may be of any general type such as the so-called Westinghouse AT relay or a motor operated contact device prevents the air valve solenoid I7 from becoming deenergized to efi'ect the closing of the air valve l3 and relea e the movable electrode from engagement with the material. In the preferred practice of my inven tion, the time delay relay 99 is of the type that is operated in one sense immediately on being energized, but must be 'maintained deenergized for an uninterrupted interval of time to be operated in the opposite-sen'se 'f Normally, the time delay relay 49'isfmaintained energized in a circuit extending from the positive terminal 5i of a Graetz connected bridge rectifier 53 supplied from the -rnain source 33 through a pair of transformers 55 and 5'I,,through a conductor 59, a conductor conductor 81, the exciting coil 69 of the time delay relay 49, a conductor 'II to the'neg'ative -ter-'" -minal 'I3--of the rectifier.

the auxiliary relay 65 is controlled by the "back switch 25.

The energization 'of El, the lowermovable contactor 63 of an auxil- U 'iarywrelay 65 which is normally deenergized,-a

When the welding current pulses'are' of relatively long duration, the premature operation of the time delay relay 49 is prevente'd by another "relay "I5 energized by the fiow of welding current.

The welding operation is started by the closing yof aimanually actuable switch 11 which is pref 'erably afoot switch. When theswitch 'I'! is closed, a circuit is completed which extends from a supply conductor I9 energized from the main sourcethroughthe transformer 55, through a conductor 8|, the exciting coil 83 of a relay 85 used-for locking the system in independently of the'foot switch; a conductor '81, the upper mov able' contactor 89 of the time delay relay 49 which --is normally in engagement with a pair of corresponding fixed'contactsi 9|, a conductor 93, a conductor95, the movable contactor 91' of the foot switch now in engagement with a pair'of corresponding fixed contacts 99, a conductor llli,

the movable contactor I93 of another rela-y I95 which prevents multipleoperation with the foot switch closed and which is engaged with correenergized from the main'source.

The lock-in'relay 85 is now operated and its sponding fixed contacts II", a conductor I08, a conductor III to the other supply conductor H3- lower movable contactor" I I5 engages correspond ing fixed contacts In bridging contacts 99 of the foot switch I1 and thus rendering the system independent of the manner of operation of the footswitch. The upper-movable contactor II9 of the lock-in' relay 85 also engages apairof cor-' responding fixed contacts I 2I and closes a cir-' cuit extending from the supply conductor II3 through a conductor I I I, the upper movable con tactor H9 of the relay 85, a conductor I23, the

exciting solenoid I Tof the air valve I3, a conto the other supply 7 II to be. welded,- After the electrode 9 has beenproperly engaged with the material, the back pressure of theair operates the back switch 25 closing a circuit extending from the supply con ductor I9 through the conductor I21, the movable.

contactor 29 of the back switch 25, a conductor I I29, the excitingcoil I3I of the auxiliary'relay controllin'g'the time delay relay 69, a conductor I33, a conductor I35, a conductor I31 to the other line conductor H3. The auxiliary relay 65 is now operated and its lower contactor.63

becoming disengaged from the corresponding fixed contacts I 39 interrupts current fiow to the exciting coil 69 of the time delay relay. However, the time delay relay 49 must remain deenergized uninterruptedly for at least a predetermined interval of the time before it operates and, therefore, no change in the circuits controlled by this relay occur for the present in spite of the operation of the auxiliary relay.

The upper movable contactor I4I of the auxiliary relay 65 engages a pair of fixed contacts I43 and closes a circuit extending from the supply conductor 19 through the conductor I21, a conductor I45, the upper movable contactor I4I of the auxiliary relay, a conductor I41, the exciting coil I49 of a relay I5I for initiating the operation of the timer for the welding current, the conductor I35, the conductor I31, to the other line conductor II 3. The timer relay I5I is now operated and its lowest movable contactor I53 engages corresponding fixed contacts I55 to connect across a resistor I51 the secondary I59 of the output transformer I6I of a circuit I63 for triggering off the timer to release the flow of welding current precisely at the desired instant. The trigger circuit I63 comprises an electric discharge device I65 which is preferably of the gaseous type having an anode I61, a cathode I69 and a control electrode I1I. The control electrode "I is connected to the cathode I69 through the output terminals I13 and I15 of a phase shifting network I11 of the usual type. The anode I61 of the discharge device I65 is connected to the cathode I69 through the primary I19 of the output transformer I6I and a portion of the windings IBI of the secondary section I83 of the transformer I85 whereby the trigger circuit is supplied. Since it is desired that the supply of welding current be triggered-off precisely, the output transformer I6I is preferably of the impulsing type in which the potential output waves of the secondary are of short duration compared to the potential supplied to the primary.

The resistor I 51 which is connected in circuit with the secondary I59 of the output transformer I6I by the lowest movable contactor I53 of the timer relay I5I is permanently connected between the control electrode I81 and the cathode I89 of the starting auxiliary discharge device M.

The circuit extends from the control electrode I81 through a grid resistor I9I, a conductor I93, the resistor I51, a conductor I95, a rectifier bridge I91, supplied from the main source, which provides a negative bias normally maintaining the starting device 4| non-conductive, a conductor I99, a rheostat 20I, to the cathode I89 of the auxiliary device.

When the timer relay I5I is operated, one of its intermediate movable contactors 203 engages corresponding fixed contacts 205, closing the anode circuit of the starting auxiliary device and incidentally also the anode circuit of the stopping auxiliary device 43. The circuit for the starting device 4I extends from the positive terminal 201 of a voltage divider 209, suppliedirom a rectifier bridge 2 energized from the main source, through a conductor 2I3, the intermediate movable contactor 203 of the relay I5I, the anode 2I5 and the cathode I89 of the starting device M, the rheostat 20I, the conductor I99 to the negative terminal 2 I 1 of the voltage divider.

After the circuit through the trigger resistor I51 is closed potential impulses are impressed across the resistor and the impulses are transferred to the control circuit of the starting device 4I. When an impulse of positive polarity is thus impressed in its control circuit, the starting device M is rendered conductive and-current flows through the rheostat 20I.

The rheostat 20I isprovided with an intermediate tap 2I9 which is connected to the control electrodes 22I of the auxiliary discharge devices 45 and 41 through which'the initiating current is supplied through the initiating electrodes 39 of the main discharge devices 35 and 31. The latter circuit extends from the intermediate tap 2I9 through a conductor 223 to the intermediate tap 225 of the secondary 221 of a compensating transformer 229 connected between the control electrodes 22I. The circuit then divides and extends through the windings of the secondary 221 and grid resistors 23I to the control electrodes 22I of the initiating devices, thence through the cathodes 232 of the initiating devices 45 and 41, conductors 233, the initiating electrodes 39 of the main discharge devices 35 and 31, the cathodes 234 of the main devices to the movable tap 235 of a voltage divider 231 interconnecting the cathodes 234, common to the divided branches of the circuit, then through a conductor 239 con nected to the movable tap 235, a conductor 24I the movable tap 243 of the voltage divider 209, whereby current is supplied to the starting device 4I, the lower winding of the voltage divider, the negative terminal 2I1 of the voltage divider, the conductor I99, to the rheostat 20I. It is to be noted that when current is not flowing through the rheostat 20I, the initiating auxiliary devices 45 and 41 are maintained non-conductive by the negative potential impressed between their control electrodes HI and their cathodes 232 by voltage divider 209 between its points 2I1 and 243. However, when the starting device M is conductive,- current flows through the rheostat 20I and a potential is impressed between the control electrodes 22 I and the cathodes 232 of the initiating devices 45 and 41 which counteracts the negative potential and the latter are rendered conductive to supply current through the initiating electrodes 39 of the main devices.

The current flow through the left-hand initiating device takes place in a circuit extending from the lower terminal of the source 33 through a conductor 245, a conductor 241, a conductor 249, a conductor 25I connected to the latter conductor 249 by the upper movable contactor 253 now engaged with cooperative fixed contacts 254 of the timer relay I5I, a current limiting resistor 255, the anode 251 and the cathode 232 of the initiating device, the conductor 233, the initiating electrode 39 of the left-hand main discharge device 35, the cathode 234 of the latter device, a conductor 259, a conductor 26I, the .primary 3| of the welding transformer 3, a conductor 263 to the upper terminal of the source 33. A similar circuit may be traced for the right-hand initiating discharge device 41 through another movable contactor 265 of the timer relay I5I which is also closed when the relay is operated.

As the alternating potential supplied from the source 33 becomes alternately positive for each of the initiating discharge devices 45 and 41 and the corresponding main discharge devices 35 and 31, the discharge devices 45 and 35 and 41 and 31 are alternately rendered conductive, half waves of current flow through the primary 3I of the welding transformer 3 and current is supplied to the material II to be welded. The supply of current continues until a variable capacitor 261 connected in parallel with the rheostat 20I through a suitable variable resistor 269 is charged.

The variable capacitor 261 is also connected between the control electrode 2' and the cathode 2'13 of the stopping auxiliary device 43 in a circuit extending from the control electrode 2H of the latter device through a grid resistor 214,

the capacitor 261, the conductor |99, the portion ing from the positive terminal 201 of the voltage divider 209 through the intermediate movable contactor 203 of the timer relay |5|, the anode 215 and cathode 213 of the stopping device 43 to the movable tap 243 of the voltage divider. The movable tap 243 of the voltage divider 209 is thus again connected to the conductor 239 leading to the movable tap 235 of the voltage divider 231 interconnecting the cathodes 234 of the main discharge devices 35 and 31 and the original negative potential is new again supplied between the control electrodes HI and the cathodes 232' of the initiating auxiliary discharge devices 2|5 and 41. The latter are then rendered non-conductive and the flow of welding current is interrupted.

Depending on the setting of the variable capacitor 291 and the variable resistor 269 in series therewith, the welding current flows for a longer or a shorter time. If it flows for a relatively short time of the order of from one-half period to one and one-half periods of the source, no further change takes place in the system until the time delay relay 49 operates since the lower movable contactor 63 of the auxiliary relay 65 is still disengaged from its fixed contacts I39. The characteristic time period of the latter relay is, of course, set at sucha value that this occurs at some time after the flow of welding current has stopped. V

If, however, the welding pulse persists for several periods of the source, as is often the case, the welding current responsive relay'15, the exciting coil 2'" of which is connected directly across the welding primary 3|, is operated and its movable contactor 219 engages corresponding fixed.

contacts 28| bridging the contacts I39 of the auxiliary relay 65 and reenergizing the time delay relay 49. The time delay relay 49 now is held in its initial energized condition as long aswelding current continues to flow. When the flow of welding current finally ceases, the welding current responsive relay I5 is deenergized and operates to open the energizing circuit for the time delay relay 49. The timing of the latter then again commences and after a predetermined interval of time it now operates, disengaging the upper movable contactor 89 from the corresponding fixed contacts 9| and engaging the lower movable contactor 283 with a pair of corresponding fixed contacts 285.

The lock-in relay 85 is deenergized by the disengagement of the upper movable contactor 89 of the time delay relay 49 and the two circuits through its contactors H5 and II 9 are opened. The circuitheld closed by the upper movable contactor 9 through the solenoid I? of the air valve I3 is thus opened and the supply of air is interrupted while the air under the electrode piston 2| is permitted to exhaust. The piston 2|, therefore, moves downwardly disengaging the movable electrode 9 from the material H to be welded. At the same time, the back pressure switch 25 is also opened and the auxiliary relay 65 deenergized. The lower movable contactor 63 of the latter relay now reengages the corresponding fixed contacts I39 reenerglzing the time delay relay 49 and operating it in preparation for another welding operation. The upper movable contactor |4| disengages its corresponding fixed contacts I43 opening the exciting circuit for the timer relay 5| and causing the latter to operate. The circuits for the starting and stopping auxiliary devices 4| and 43 and for the initiating devices 45 and 41 are now open. In addition, an intermediate movable contactor 281 of the timer relay engages corresponding fixed contacts 289 to provide a discharging circuit for the variable capacitor 261.

Since the system is manually operable, it may happen that the operator actuates the foot switch I! accidentally just after the time delay relay 49 has operated and has been reverted to the energized condition. To prevent the flow of deleterious welding current by the accidental operation of the switch 11, an electronic timing relay 29| is provided. This relay is described in detail in my Patent No. 2,008,413 for Electric discharge apparatus, dated July 16, 1935, and assigned to the assignee of the present application.

The operation of the relay 29| is initiated by the engagement of the lower movable contactor 283 of the time delay relay '59 with its fixed contacts 285. A circuit is thus closed which extends from the upper terminal of a secondary section 293 of the transformer 295 whereby the electronic relay is supplied, through a conductor 291, a conductor 299, the closed contactor 283 of the time delay relay 49, a conductor 299, a conductor 30L the exciting coil 303 of a grid control relay 305 in the electronic timer circuit, a conductor 30! to the lower terminal of the secondary section. The grid control relay 305 operates, reversing the connections of a capacitor 309 which is normally charged through the grid circuit of the discharge device 3|| of the electronic timer 29| to a potential such that it biases the discharge device to a non-conductive condition. By the reversal of the capacitor connections, the eifect of the capacitor potential is reversed and the.

discharge device 3| is rendered conductive.

A circuit 'is now closed which extends from the upper terminal of the secondary section 293 through the discharge device 3, the exciting coil 3|3 of the main relay 3|5 of the timer, the conductor 301, to the lower terminal of the sec ondary section. The latter relay 3|5 is energized and immediately operates and it remains in its operated condition as long as the capacitor continues sumciently charged to maintain the discharge device 3 energized. This condition is controlled by a plurality of resistors 3H of diflerent magnitudes one of which may be selected and connected through contactor 3|8 to the capacitor 309 for any desired timing when the capacitor connections are reversed. The ca- 'pacitor 309 discharges through the selected resisto'r' 3|? and when it has discharged the discharge device is rendered non-conductive and the relay is deenergized. The relay 3|5 is, more- 7 over, provided with an upper movable contactor 3! 9 which bridges the lower contacts 285 of the time delay relay 49 and thus maintains the grid control relay 305 energized and the electronic timer 29l inoperative after the time delay relay has been reenergized.

The lower movable contactor 32! of the main timer relay 3l5 on the other hand engages corresponding fixed contacts 323 and closes a circuit through the exciting coil 325 of the multipleoperation preventing relay I05. The latter is energized and its movable contactor I03 disengages the corresponding fixed contacts I01, maintaining the circuits through the foot switch 11 open. The latter condition persists for a time period determined by the electronic timer 2! which is adjustable at will by simply selecting the proper resistance through which the capacitor is discharged. Supply of welding current to the system by accidental operation of the foot switch 1'! is thus prevented for at least a predetermined interval of time. After the electronic timer completes its operation, the relay I05 is again deenergized closing the circuits through the foot switch 11 and the whole system is reset for another operation.

Although I have shown and described certain specific embodiments of my invention, I am fully aware that many modifications thereof are possible. My invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. Apparatus for welding a material with a system including electrodes through which current is to be transmitted to said material comprising means for moving said electrodes into engagement with said material and holding them in the said engagement, means for supplying an impulse of current to weld said material, a first means to prevent the disengagement of said electrodes from said material until the termination of said impulse when said impulse is of relatively long duration said first means being of the type that fails to function if said impulse is of relatively short duration and a second means for preventing the disengagement of said electrodes from said material when said impulse is of relatively short duration.

2. For use with apparatus for welding a material with a welding system including electrodes through which current is to be transmitted to weld said material and means for moving said electrodes into engagement with said material and holding them in the said engagement, the combination comprising means for supplying an impulse of current to weld said material, a first means to prevent the disengagement of said electrodes from said material until the termination of said impulse when said impulse is of relatively long duration said first means being of the type that fails to function if said impulse is of-relatively short duration and a second means to prevent the disengagement of said electrodes from said material when said impulse is of relatively short duration.

3. For use with apparatus for welding a material with a welding system including electrodes through which current is to be transmitted to weld said material and means for moving said electrodes into engagement with said material and holding them in the said engagement, the combination comprising means for supplying an impulse of current to weld said material, a first means including electromagnetic actuatingmeans to prevent the disengagement of said electrodes from said material until the termination of said impulse when said impulse is of sufficiently long duration to permit actuation of said first means, and a second means to prevent the disengagement of said electrodes from said material whenterial with a welding system including electrodes through which current is to be transmitted to Weld said material and means for moving said electrodes into engagement with said material and holding them in the said engagement, the combination comprising means for supplying an impulse of current to weld said material, a first means responsive to the flow of said impulse of current to prevent the disengagement of said electrodes from said material until the termination of said impulse when said impulse is of predetermined duration, and a second means to prevent the disengagement of said electrodes from said material when said impulse is of shorter duration than said predetermined duration.

5. For use with apparatus for Welding a material with a welding system including electrodes through which current is to be transmitted to weld said material and means for moving said electrodes into engagement with said material and holding them in the said engagement, the combination comprising means for supplying an impulse of current to weld said material, a first means to prevent the disengagement of said electrodes from said material until the ,termination of said impulse when said impulse is of relatively long duration and a second means to prevent the disengagement of said electrodes from said material when said impulse is of relatively short duration, the actuation of said first means operating to prevent the actuation of said second means.

6. Apparatus for welding a material with a system including electrodes through which current is to be transmitted to said material comprising means including a manually actuable device for moving said electrodes into engagement with said material and holding them in the said engagement, means for supplying an impulse of current to weld said material, a first means to prevent the disengagement of said electrodes from said material until the termination of said impulse when said impulse is of relatively long duration said first means being of the type that fails to function if said impulse is of relatively short duration and a second means for preventing the disengagement of said electrodes from said material when said impulse is of relatively short duration, said first and said second means operating to perform their respective functions regardless of whether or not said manually actuable device remains actuated after having once being actuated.

'7. Apparatus for supplying current to a load from a source, comprising valve means interposed between said source and said load operating to limit the current fiow to pulses persisting for a predetermined interval of time that may be set at will, a device to be actuated in one sense prior to the initiation of each one of said pulses and to be actuated in the opposite sense after the termination of each one of said pulses and prior to the initiation of a succeeding pulse, a first means to prevent the actuation of said device in said opposite sense until after the termination of a pulse if said pulse persists for at least a first predetermined interval of time, said first -means being of the type that, when acting alone,

it functions to prevent the actuation of said device in said opposite'sense for a second predetermined interval of time after the initiation of said pulse and a second means to prevent the actuation of said device in said opposite sense until after the termination of said pulse if said pulse persists foran interval of time larger than said first predetermined interval.

8. Apparatus for supplying current to a load from a source, comprising valve means interposed between said source and said load operating to limit the current flow to pulses persisting for a predetermined interval of time that may be set atwill, a device to be actuated in one sense prior to the initiation of each one of said pulses and to be actuated in the opposite sense after the termination of each one of said pulses and prior to the initiation of a succeeding pulse, a first electromagnetic relay requiring actuation for an uninterrupted interval of time of predetermined length before it is operated, functioning when operated to actuate said device in the opposite sense, the actuation of said relay commencing prior to the fiow of a current pulse following the actuation of said device in said one sense, and a second electromagnetic relay connected to be actuated by the flow of said current pulses, said second relay when actuated operating to interruptthe actuation of said first relay.

9. Apparatus for supplying current to a load from a source, comprising valve means interposed between said source and said load operating to limit the current flow to pulses persisting for a predetermined interval of time that may be set at will, a device to be actuated in one sense prior to the initiation of each one of said pulses and to be actuated in theopposite sense after the termination of each one of said pulses and prior to the initiation of a succeeding pulse, a first electromagnetic relay requiring actuation for an uninterrupted interval of time of predetermined length before it is operated, functioning when operated to actuate said device in the opposite sense, the actuation of said relay commencing prior to the flow of a current pulse following the actuation of said device in said one sense, and a second electromagnetic relay connected to be actuated by the flow of said current pulses, said second relay when actuated operating to interrupt the actuation of said first relay, the length of said interval of time being substantially greater than the time required for operation of said second relay after it is actuated by the flow of said current pulse.

10. Apparatus for supplying current to a load from a source, comprising valve means interposed between said source and said load operating to limit the current 'fiow to pulses persisting for a predetermined interval of time that may be set at will, a device to be actuated in one sense prior to the initiation of each one of said pulses and to be actuated in the opposite sense after the. termination of each one of said pulses and prior to the initiation of a succeeding pulse, a first electromagnetic relay requiring actuation for an uninterrupted interval of time of predetermined length before it is operated, functioning when operated to actuate said device in the opposite sense, manually actuable means for first actuating said device in said one sense and before the flow of a current pulse, commencing the actuation' of said first relay and a second electromagnetic relay connected to be actuated by the fiow of said current pulses, said second relay when actuated operating to interrupt the actuation of said first relay.

11. Apparatus for supplying current to a load from a source, comprising valve means interposed between said source and said load operating to limit the current fiow to pulses persisting for a predetermined interval of time that may be set at will, a device to be actuated in one sense prior to the initiation of each one of said pulses and to be actuated in the opposite sense after the termination of each one of said pulses and prior to the initiation of a succeeding pulse, a first electromagetic relay requiring actuation for an'uninterrupted interval of time of predetermined length before it is operated, functioning when operated to actuate said device in the opposite sense, manually actuable means for first actuating said device to operate in said one sense and then, before the flow of a current pulse, commencing the actuation of said first relay, a second electromagnetic relay connected to be actuated by the fiow of said current pulses, said second relay when actuated operating to interrupt the actuation of said first relay and means to prevent the reactuation of said device in said one sense for at least a length of time predeterminable at will regardless of whether or not said manually actuable means is actuated.

12. For use with apparatus for welding a material with a welding system including electrodes through which current is to be transmitted to weld said material and means for moving said electrodes into engagement with said material and holding them in the said engagement, the current flow through said material at times persisting only for a short interval of time of the order of a cycle of a 60 cycle source, the combination comprising means for initiating the supply of an impulse of current to weld said material and means activated by said initiating means substantially when the flow of said welding impulses is initiated, and operating to disengage said electrodes from said material a predetermined interval of time after said impulse has been terminated suificiently long to enable the weld to chill and harden when said current flow persists only for said short interval. Y 13. For use with apparatus for welding a material with a welding system including electrodes thru which current is to be transmitted to weld said material and means for moving said electrodes into engagement with said material and holding them in the said engagement, the combination comprising means for initiating the supply of an impulse of current tolweld said material and means activated substantially when the flow of said welding impulse is initiated by said initiating means, and operating to disengage said electrodes from said material a predetermined interval of time, sufiiciently long to enable the weld to chill and harden, after said impulse has been terminated, said activated means including means for preventing the premature operation thereof when said impulse is of substantial length.

14, Apparatus according to claim 13 characterized by the fact that the activated means includes a time delay relay and a relay operated in response to the flow of welding current which lengthens the delay period of the time delay relay substantially by the time length of the welding impulse.

JOHN W. DAWSON. 

